1. Field of the Invention
The present invention relates to an insulating communication circuit and transmission of a communication signal by an insulating circuit between a low-voltage system and a high-voltage system having different grounds.
2. Description of the Related Art
In electric vehicles such as electric cars and hybrid cars, a DC voltage from a high-voltage DC power supply is converted to an AC voltage, e.g. a three-phase AC voltage, by an inverter and an electric machine as a load is rotated by this three-phase AC voltage to thereby obtain the drive source of vehicle travelling. Furthermore, a hybrid vehicle is a car that employs a motor in addition to an engine as the drive source of its travelling, and the motor is driven to assist the engine and carry out regenerative braking of the motor.
The inverter has switch elements and free-wheel diodes connected in anti-parallel to the switch elements on the high side and the low side for the U-, V-, and W-phases. The inverter carries out switching of the switch elements based on a PWM system to thereby obtain the three-phase AC voltage.
Due to the switching-ON/OFF of the switch elements, the phase voltage changes, and a common mode current flows to the vehicle body (body ground (hereinafter, the low-voltage system ground)) via the parasitic capacitance between the positive line of the high-voltage DC power supply and the low-voltage system ground, the parasitic capacitance between the ground connected to the negative electrode of the high-voltage DC power supply (hereinafter, the high-voltage system ground) and the low-voltage system ground, a Y-capacitor provided between the positive line of the high-voltage DC power supply and a low-voltage system ground line and the low-voltage system ground, the parasitic capacitance between motor coils and the low-voltage system ground, and the parasitic capacitance between three-phase power supply lines interconnecting the inverter and the motor coils and the low-voltage system ground.
Furthermore, due to the switching-ON/OFF of the switch elements, the connecting parts between the switch elements on the low side and the high-voltage system ground line connected to the negative electrode of the high-voltage DC power supply are in such a state that the potential of the high-voltage system ground easily changes due to a current flow attributed to commutation at the time of the switching or the like. This potential change of the high-voltage system ground is transmitted to other high-voltage system grounds that share the ground, so that the potential of the high-voltage system ground changes.
On the other hand, electrical loads such as an ECU typified by an engine ECU, a headlight, car audio equipment, and a door mirror heater operate based on power from a 12-V battery whose negative electrode is connected to the vehicle body. The components whose ground is the vehicle body are referred to as the low-voltage system. In electric vehicles, the high-voltage system whose ground is the high-voltage system ground and the low-voltage system exist in a mixed manner.
In order to drive the motor with the optimum torque based on the engine torque and so on, a communication signal of the target torque and so on and a gate signal need to be transmitted from the low-voltage system to the high-voltage system. For this signal transmission from the low-voltage system to the high-voltage system, a pulse transformer is used because a photocoupler or the like is expensive.
The primary winding and the secondary winding of the pulse transformer are insulated from each other by covering of the windings or an inter-phase tape, and the primary winding and the secondary winding have coupling capacitance because they are very close to each other. If the potential of the high-voltage system ground of the secondary winding of the pulse transformer changes, the voltage between the primary winding and the secondary winding changes.
The time period of this voltage change is on the order of microseconds or shorter. Even with low coupling capacitance, this voltage change causes a common mode current to flow from the secondary winding to the primary winding or from the primary winding to the secondary winding via the coupling capacitance between the primary winding and the secondary winding. The flow of the common mode current generates voltage across the primary winding and the secondary winding due to the coil inductance and the coil resistance of the primary winding and the secondary winding. As a result, the voltage across the secondary winding changes, so that noise due to the common mode current is added to the communication signal that should be transmitted.
In a related art, for the pulse transformer that transmits a signal from the low-voltage system to the high-voltage system, the noise added to the communication signal due to the common mode current is removed by a filter. Examples of the related art relating to the common mode current include Japanese Patent Laid-Open No. 2006-33647. In the technique disclosed in Japanese Patent Laid-Open No. 2006-33647, the common mode current is detected by the common mode noise detection circuit 8 and the common mode noise cancel circuit 9 is driven by the detected electric signal to thereby cancel the common mode current.
However, the related art involves a problem that the communication signal is delayed because the common mode current flows via the coupling capacitance of the pulse transformer and the noise added to the communication signal is removed by using the filter. Furthermore, as another problem, the noise can not be sufficiently removed when the level of the noise is high. In addition, the above-described patent document involves a problem that the cost is high because the noise cancel circuit for cancelling noise is necessary.
The present invention is made in view of the above-described problems and an object thereof is to provide an insulating communication circuit that has solutions to the problems such as signal delay and insufficient noise removal and allows cost reduction without using the noise cancel circuit and the filter.